Composite cathode

ABSTRACT

A composite cathode for a battery is disclosed. In addition, a method is disclosed for producing said composite cathode which can be configured in a battery. The method comprises steps or (1) preparing a solution or an organic disulfide compound in 2-pyrrolidone or its derivative, (2) adding polyaniline to the solution to dissolve, thereby obtaining a homogeneous liquid, and then (3) removing at least a part of the 2-pyrrolidone or its derivative from the homogeneous liquid to obtain a solid product wherein the organic disulfide compound and the polyaniline are homogeneously mixed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite electrode which has a wideapplication in various electrochemical devices such as batteries,electrochromic display devices, sensors and memories. More particularly,the present invention provides a composite electrode which comprises aplurality of organic compounds, or a plurality of organic and inorganiccompounds. It is further concerned with a method for producing such anelectrode as well as a lithium secondary battery using such an electrodeas its cathode.

2. Description of the Related Art

Since conductive polyacetylene was discovered by Shirakawa et al. in1971, the use of this conductive polymer as an electrode material hasbeen extensively studied. This is because electrochemical devices suchas lightweight batteries with a high energy density, electrochromicdisplay devices having a large area and rapid coloring and decoloringcharacteristics, and biochemical sensors using minute electrodes can beexpected to be realized by using the conductive polymer as the electrodematerial. The polyacetylene however has some disadvantages for practicaluse in the electrodes because of its chemical instability, and hence,research has since been directed to other π electron conjugatedconductive polymers, which are relatively stable, such as polyaniline,polypyrrole, polyacene and polythiophene. Lithim secondary batteriesusing these polymers for their cathodes have already been developed.

Separate from this, disulfide compounds have been proposed as organicmaterials which may realize a high energy density in electrochemicaldevices, for example, disclosed in Dejoughe et al., (U.S. Pat. No.4,833,048). Typical disulfide compounds are represented, in their mostbasic form, by the formula: R--S--S--R (wherein, R represents analiphatic or an aromatic organic residue, and S represents a sulfuratom). The S--S bond may be cleaved by electrolytic reduction to form asalt with a cation (M⁺) in an electrolytic cell. The salt is representedby the formula of R--S⁻ ·M⁺. The salt of R--S⁻ ·M⁺ may be returned tothe original R--S--S--R by electrolytic oxidation. A metal-sulfur typerechargeable battery constructed by combining a disulfide type compoundwith a metal (M) which supplies and captures cations (M⁺) is proposed inDejoughe et al. As described therein, it is expected that such asecondary battery will have an energy density of 150 Wh/kg or more,which is much more than that of the conventional secondary batteries.

These conductive polymer electrodes, comprising at least one ofpolyaniline, polypyrrole, polyacene and polythiophene, take in anionsexisting in the electrolyte as well as cations upon an electrodereaction while they are charged or discharged, and hence the electrolytein the battery not only functions as a transfer medium for ionconduction but also participates in the battery reaction. Therefore, itis required that the electrolyte is supplied to the battery in an amountcorresponding to the battery capacity. Thus, it has some disadvantagesthat the energy density of the battery is limited that much to the rangeof about 20 to 50 Wh/kg, which is nearly half of that of theconventional secondary batteries such as nickel-cadmium or lead-acidbatteries.

On the other hand, as Dejoughe et al. reported in J. Electrochem. Soc.,Vol. 136, No. 9, pp. 2570 to 2575 (1989), the difference betweenoxidation potential and reduction potential of the organic disulfidecompound is as large as 1 volt or more in an electrolysis of, forinstance, a compound [(C₂ H₅)₂ NCSS⁻ ]₂. According to the electrodereaction theory, the electron transfer of the disulfide compoundproceeds extremely slowly. Because of this, it is rather difficult toobtain a rechargeable battery which may provide a higher current outputof, for instance, 1 mA/cm² or more at room temperature. Therefore, thereis a disadvantage that the operation of a battery comprising anelectrode of disulfide compound is limited to that at a high temperaturein the range of from 100° C. to 200° C., where the electron transfer canproceed faster.

SUMMARY OF THE INVENTION

The present invention-provides a novel composite electrode, whichovercomes the above-discussed and numerous other disadvantages anddeficiencies of the prior art, and is excellent in reversibility beingcapable of charging and discharging at a large current at roomtemperature, while maintaining tile above-mentioned preferable featuresof high energy density held by the organic disulfide compounds. It alsoprovides a method of producing such electrodes.

The method of producing a composite electrode in accordance with thepresent invention comprises the steps of:

(1) dissolving an organic disulfide compound which contains at least onesulfur-sulfur bond or at least two thiolate or thiol groups in2-pyrrolidone or its derivative represented by the formula: ##STR1##where R represents a hydrogen atom or an alkyl group, to form asolution, the sulfur-sulfur bond being cleaved when electrolyticallyreduced to form thiolate groups or thiol groups and the sulfur-sulfurbond being regenerated when the thiolate or thiol groups areelectrolytically oxidized,

(2) adding polyaniline to the solution to dissolve the polyaniline,thereby obtaining a homogeneous liquid, and

(3) removing at least a part of the 2-pyrrolidone or its derivative fromthe homogeneous liquid to obtain a solid product wherein the organicdisulfide compound and the polyaniline are homogeneously mixed.

The thiolate group is represented by --SMe where Me is a metal atom, andthe thiol group is represented by --SH.

In the method mentioned above, the step (3) may optionally be precededby a step of applying the homogeneous liquid obtained in the previousstep on a substrate to form a layer.

The above-mentioned method may further comprise the steps of:

(1) pulverizing the solid product to obtain a powder wherein the organicdisulfide compound and the polyaniline are homogeneously mixed, and

(2) molding the powder with the application of pressure to obtain acomposite electrode of film shape or plate shape.

The above-mentioned method may further comprise the steps of:

(1) pouring the 2-pyrrolidone or its derivative solution on apolyaniline film to be flown or spread thereover, and

(2) heating the polyaniline film, having a layer of the solutionthereon, in a vacuum or in an inert gas atmosphere.

In the above-mentioned method, the polyaniline film is preferablyobtained by applying an 2-pyrrolidone or its derivative solution ofpolyaniline on a substrate and removing the 2-pyrrolidone or itsderivative contained in the applied solution.

Further, the above-mentioned method may also comprise the steps of:

(1) adding a metal oxide powder to the homogeneous liquid obtained inthe previous step to obtain a mixture wherein the metal oxide powder ishomogeneously dispersed in the liquid, and

(2) removing at least a part of the 2-pyrrolidone or its derivative fromthe mixture to obtain a solid product wherein the organic disulfidecompound, the polyaniline and the metal oxide powder are homogeneouslymixed.

The above-mentioned method may further comprise the steps of:

(1) applying the mixture wherein the metal oxide powder is homogeneouslydispersed in the homogeneous liquid obtained in the previous step on asubstrate to form a layer, and

(2) removing at least a part of the 2-pyrrolidone or its derivative fromthe applied layer to obtain a solid product film wherein the organicdisulfide compound, the polyaniline and the metal oxide arehomogeneously mixed.

The above-mentioned method may further comprise the steps of:

(1) pulverizing the solid product film to obtain a powder wherein theorganic disulfide compound, the polyaniline and the metal oxide arehomogeneously mixed, and

(2) molding the powder with the application of pressure to obtain acomposite electrode of film shape or plate shape.

In the above-mentioned method, the polyaniline is preferably apolyaniline of de-doped and reduced state, and the metal oxide ispreferably at least one of transition metal oxide selected from thegroup consisting of LiCoO₂, V₆ O₁₃, LiMn₂ O₄, V₂ O₅ and LiNiO₂.

In the above-mentioned method, the organic disulfide compound is atleast one selected from the group consisting of dithioglycol (HSCH₂ CH₂SH), 2,5-dimercapto-1,3,4-thiadiazole (C₂ N₂ S(SH)₂), 2,4,6-trithiol (C₃H₃ N₃ S₃), 7-methyl-2,6,8-trimercaptopurine (C₆ H₆ N₄ S₃), and4,5-diamino-2,6-dimercaptopyrimidine (C₄ H₆ N₄ S₂).

The composite electrode according to the present invention comprises:

an organic disulfide compound, and

polyaniline.

In the above-mentioned composite electrode, the polyaniline may behomogeneously mixed with the organic disulfide compound, or mayalternatively be in a film, on which a layer containing the disulfidecompound is laminated.

The above-mentioned composite electrode may optionally comprise2-pyrrolidone or its derivative.

The electrode works regardless of the presence or absence of the2-pyrrolidone or its derivative.

The above-mentioned composite electrode may further comprise a metaloxide powder which is homogeneously mixed with the polyaniline andorganic disulfide compound.

While the novel features of the present invention are set fourth in thepreceding, the invention, both as to organization and content, can befurther understood and appreciated, along with other objects andfeatures thereof, from the following detailed description and examples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a composite electrode, which combines anorganic disulfide compound with polyaniline, or the organic disulfidecompound and polyaniline with a metal oxide powder, by utilizing aphenomenon that the disulfide compound and polyaniline can give ahomogeneous mixture if they are dispersed with a medium comprising2-pyrrolidone or its derivative.

An organic disulfide compound (hereinafter referred to "S--S compound"),2-pyrrolidone or its derivative and polyaniline each has a certainsolubility with one another at room temperature and, when combined, forma homogeneous composite product having a certain adhesive property. Inthe composite product, the S--S compound functions as an electrodereaction substance, and gives an ion conductivity to the compositeproduct. The polyaniline functions as an electrode reaction substance aswell as a catalyst for an electrode reaction of the S--S compound, andgives an electronic conductivity to the composite product. Thus, whenthe above-mentioned composite product is employed for the electrodematerial, a difference between oxidation potential and reductionpotential generated by the S--S compound decreases to 0.1 volt orsmaller from the conventional value of 1 volt or larger. Therefore, theelectrode reaction is promoted and a preferable ion conductive andelectron conductive network is formed in the composite electrode, andhence, a charge and discharge at a large current can be made possibleeven at room temperature.

Although an introduction of an electrode catalyst into an electrode ofan organic disulfide compound is discussed in the above-mentioned U.S.Pat. No. 4,833,048 or in J. Electrochem. Soc., Vol. 136, p.2570-2575(1989), only an organic metal compound is disclosed as the electrodecatalyst. There is no concrete description on the advantage of theintroduction of the electrode catalyst.

Although the solubility of polyaniline in 2-pyrrolidone or itsderivative is as small as several percent by weight, the solubilityincreases up to an equimolar amount for the 2-pyrrolidone or itsderivative if the S--S compound is present together with thepolyaniline. The S--S compound and 2-pyrrolidone or its derivative arereadily soluble with each other and give a homogeneous solution havingsome viscosity even in a mixture of an equimolar amount. It is believedthat an association may take place among an --SH group in the S--Scompound, a ═C═O group in the 2-pyrrolidone or its derivative and an--NH group in the polyaniline in its phenylene diamine structure oneanother through a hydrogen bond or the like, and results in ahomogeneous composite product. The --NH group is produced in thepolyaniline when it constitutes the phenylene diamine structure in itsreduced and de-doped state.

In the production method of the composite electrode in accordance withthe present invention, an electrode, wherein the content of polyanilinein one side face thereof is higher than that in the other side face, canbe produced. Such an electrode is produced by coating a 2-pyrrolidone orits derivative, solution of the S--S compound on a polyaniline film,which has previously been prepared, and then by heating the whole in avacuum or in an inert atmosphere. When the thus prepared compositeelectrode is employed in an electrochemical device such as a battery, adevice having a large output current can be produced. For such anelectrochemical device, by arranging the electrode such that the face ofa higher content of the polyaniline having a high electronicconductivity is in contact with a current collector, and that the faceof a higher content of the S--S compound having a high ionicconductivity is in contact with an electrolyte. By arranging thecomposite electrode in the above-mentioned manner, a preferable contactbetween the electrode and the electrolyte and that between theelectrolyte and the current collector can be ensured in both theelectronic and ionic points of view.

In particular, it is possible to obtain a lithium secondary batteryhaving a high energy density and a large output current, by employingmetal lithium, a lithium alloy such as lithium-aluminum orlithium-manganese, or a carbon material, a metal sulfide, a metal oxideor an electrically-conductive polymer which can reversibly intercalatingor deintercalating lithium as its anode, and by employing the compositeelectrode produced in accordance with the present invention as itscathode.

In a composite electrode produced in accordance with the presentinvention by employing the organic S--S compound, polyaniline and metaloxide in combination, a sulfur-metal ion bond of the S--S compounddissociates to form a S--S compound anion, and this anion dopes into asite of nitrogen atom in the polyaniline in lieu of the electrolyteanion during a charging (oxidation) process of the composite electrode,and hence, the polyaniline is doped without consuming the electrolyteanion. Further, the S--S compound polymerizes through a sulfur-sulfurbond and forms a side or secondary chain against the main or primarychain in the polyaniline. With the progress of doping of thepolyaniline, an electronic conductive path in the composite electrodedevelops or grows to form a network, and promotes a charging reaction(oxidation) of the metal oxide. The oxidation of the metal oxideproceeds with the de-doping of the metal cation and the de-doped metalcation deposits on the anode in its metal state or is doped in theanode, and thus no electrolyte is consumed.

On the contrary, during a discharging (reduction) process of thecomposite electrode, a doping is made on the metal-cation which haspreviously been formed by dissolution of the anode or by de-doping ofthe anode, maintaining the network of the electronic conductive path,which has previously been formed with the polyaniline and the S--Scompound by the dissolution of the anode or by the de-doping of theanode. Next, a de-polymerization of the polymerized S--S compoundoccurs, and the S--S compound anion de-polymerized to its monomer statefinally de-dopes from the polyaniline. That is, no electrolyte isconsumed also in the discharging process and the electronic conductivepath developed in the network state promotes the discharging of themetal oxide.

Therefore, the composite electrode produced in accordance with thepresent invention can obtain a higher service capacity by 1.5 times or 2times as compared with an electrode solely composed of polyaniline or anelectrode comprised of polyaniline and metal oxide in combination.Further, since the electronic conductive path in the network state isformed with the polyaniline and the S--S compound, the oxidizing andreducing reactions of the metal oxide are promoted and result in a smallpolarization value, and a larger current can be flown therethrough ascompared with the electrode comprised of polyaniline and metal oxide incombination.

In producing a composite electrode in accordance with the presentinvention, wherein 2-pyrrolidone or its derivative is employed as themedium, and polyaniline in a de-doped and reduced state is employed asthe polyaniline, it is possible to obtain a composite electrode whereinmetal oxide is homogeneously dispersed only by a mechanical dispersingand mixing means. That is, in 2-pyrrolidone or its derivative, thepolyaniline in a de-doped and reduced state and the S--S compound form acomposite product excellent in their solubility and thus give ahomogeneous solution. This solution can be diluted with 2-pyrrolidone orits derivative in an arbitrary proportion.

Therefore, a composite electrode of a uniform composition can beobtained by homogeneously dispersing metal oxide powder in 2-pyrrolidoneor its derivative by only means of mechanical dispersion and mixing,then by molding the dispersion in a suitable shape if required andthereafter by removing at least a part of the 2-pyrrolidone or itsderivative contained in the dispersion.

In the prior art production of the composite electrode comprised ofpolyaniline and metal oxide in combination, it has been conventional toobtain a composite product of the polyaniline and the metal oxide bypolymerizing through an electrolytic or chemical oxidation an anilinemonomer in a state of a solution with dispersed metal oxide powder. Inthe case of polyaniline is present solely, only about 0.1. g ofpolyaniline can however be dissolved in 1 g of 2-pyrrolidone or itsderivative. In contrast to this, 0.5 g or more of polyaniline can bedissolved in 1 g of the 2-pyrrolidone or its derivative in the presenceof the S--S compound together with the polyaniline.

The S--S compound monomer used in the present invention can beexemplified by at least one of the compounds represented by the generalformula (R(S)_(y))_(x), which are disclosed in the United State PatentNo. 4,833,048. In the general formula, R represents an aliphatic residueor an aromatic residue, S represents sulfur, y represents an integer ofone or larger and n represents an integer of two or larger. The compoundcan specifically be exemplified by dithioglycol represented by HSCH₂ CH₂SH (hereinafter referred to "DTG"), 2,5-dimercapto-1,3,4-thiadiazolerepresented by C₂ N₂ S(SH)₂ (hereinafter referred to "DMcT"),s-triazine-2,4,6-trithiol represented by C₃ H₃ N₃ S₃ (hereinafterreferred to "TTA"), 7-methyl-2,6,8-trimercaptopurine represented by C₆H₆ N₄ S₃ (hereinafter referred to "MTMP"),4,5-diamino-2,6-dimercaptopyrimidine represented by C₄ H₆ N₄ S₂(hereinafter referred to "DDPy") or the like. Any of commerciallyavailable products of these compounds can be employed as it is.

A commercially available reagent of the certified grade may be employedas the 2-pyrrolidone or its derivative. These can be exemplified as2-pyrrolidone wherein R is hydrogen atom, N-methyl-2-pyrrolidone whereinR is methyl group, N-ethyl-2-pyrrolidone wherein R is ethyl group,N-n-butyl-2-pyrrolidone wherein R is n-buthyl group, and the like,having an alkyl group of straight chain or branched chain. Particularlypreferred 2-pyrrolidone or its derivative is one having a low molecularweight of 2-pyrrolidone, N-methyl-2-pyrrolidone orN-ethyl-2-pyrrolidone.

As the polyaniline, any of polymerized products obtained by oxidizinganiline in an electrolytic oxidation process or a chemical oxidationprocess can be employed. From a point of view of its solubility, it ispreferable to use a de-doped and reduced state polyaniline. As suchpolyaniline, ANILEAD (Registered trade name, polyaniline,conductivity=10⁻⁹ S/cm, density=1.3 g/cc), is available from Nitto DenkoCorp., Japan.

The composite formation of the S--S compound, 2-pyrrolidone or itsderivative and polyaniline can be performed by first mixing the S--Scompound with 2-pyrrolidone or its derivative to obtain a viscous liquidand then adding polyaniline powder to the liquid thus obtained todissolve therein. Inn particular, a composite electrode film whose oneside face is abundant in polyaniline and the other side face is abundantin the S--S compound can be obtained in the following manner. That is, apolyaniline film is first prepared in a film state on a substrate, and a2-pyrrolidone or its derivative solution containing the S--S compound iscoated over the film thus obtained, and then the whole is heated in avacuum or an inert gas atmosphere at a temperature ranging from 60° C.to 100° C. The polyaniline in a film state can be obtained by depositingpolyaniline on the substrate by an electrolytic polymerization. Suchpolyaniline film can alternatively be obtained by synthesizingpolyaniline by an electrolytic polymerization or a chemicalpolymerization, and then pouring a solution prepared by dissolving thepolyaniline in 2-pyrrolidone or its derivative, on a substrate to beflown or spread thereover; thereafter, the 2-pyrrolidone or itsderivative contained in the poured solution is removed. Preferablemixing ratio of the S--S compound, 2-pyrrolidone or its derivative andpolyaniline is somewhere in 1 mole of the S--S compound to 0.5-5 molesof 2-pyrrolidone or its derivative and to 0.05-5 moles of polyaniline.

Any of the oxides of transition metals or their complex or double oxidescan be used as the metal oxide though, those compounds having anelectrochemical equivalent which is equal to that of polyaniline of 150mAh/g or larger are preferable. The preferred transition metal oxide canbe exemplified by LiCoO₂ (electrochemical equivalent=140-160 mAh/g), V₆O₁₃ (electrochemical equivalent=160-230 mAh/g), LiMn₂ O₄(electrochemical equivalent=100-120 mAh/g), V₂ O₅ (electrochemicalequivalent=130-150 mAh/g) and LiNiO₂ (electrochemical equivalent=140-220mAh/g). These transition metal oxide may be employed in a powdery statewith a mean particle size ranging from 1 μm to 10 μm. A plurality of thetransition metals may be contained in the transition metal oxide. Forinstance, a double oxide of LiCoO₂ type whose Co is partly substitutedby Mn, Ni or Fe, or a double oxide of V₆ O₁₃ type whose V is partlysubstituted by W can also be used.

The composite formation of polyaniline, the S--S compound and metaloxide is performed in the following manner. First, the S--S compound isdissolved in 2-pyrrolidone or its derivative to obtain a viscous liquidand polyaniline powder is added to the liquid to be dissolved therein.If required for the dissolving, the added liquid is heated. And, ifrequired, 2-pyrrolidone or its derivative is further added thereto todilute the mixture. Then, a slurry which has previously been prepared bydispersing the powder of metal oxide in 2-pyrrolidone or its derivativeis added to the diluted solution and the whole is uniformly dispersed ina homogenizer. The dispersion thus obtained is flown or spread over asubstrate such as glass flat dish or carbon film, and the whole isheated under a reduced pressure to remove at least a part of the2-pyrrolidone or its derivative contained in the dispersion to obtain acomposite electrode. A preferred mixing ratio of polyaniline and theS--S compound is somewhere in 1 mole of the polyaniline to 0.5-5 molesof the S--S compound. A preferred mixing ratio of polyaniline and themetal oxide is somewhere in 1 part by weight of the polyaniline to 0.5-5parts by weight of the metal oxide.

In addition to an alkali metal ion and alkaline earth metal iondisclosed in the above mentioned U.S. patent, a proton can also beemployed as a metal ion which is required when the S--S compound isreduced to form a salt.

In the case of employing lithium ion as the alkali metal ion inparticular, a lithium secondary battery having a high energy densitywhich exceeds 100 Wh/kg at a cell voltage of about 3 V can be configuredwith an electrode which can supply or capture lithium ion and anelectrolyte which can transfer the lithium ion, in the following manner.That is, as the electrode which can supply or capture the lithium ion,there is employed a metal lithium or a lithium alloy such aslithium-aluminum. As the electrode which can reversibly intercalate ordeintercalate the lithium ion, there is employed a carbon material suchas graphite, a metal sulfide, a metal oxide, or an organic semiconductormaterial such as polyacene.

In an alternative case of employing proton, a secondary battery having acell voltage ranging from 1 V to 2 V can be configured by employing ahydrogen storage alloy such as LaNi₅ or its hydride as an electrodewhich can supply or capture the protons, and a proton-conductiveelectrolyte.

In addition to the above-defined components, the composite electrode ofthe present invention can include an electric conductivity enhancingagent such as carbon, a shape-retaining agent or reinforcing agent suchas a synthetic rubber, a resin, or a ceramic powder. Further, for thepurpose of improving the ion conductivity of the composite electrode,any of a solid electrolyte, a polymer electrolyte, an organicelectrolyte can be incorporated.

The present invention may be further understood by reference to thefollowing non-limiting examples and comparative examples.

EXAMPLE 1

One point five (1.5) g (0.01 mole) of 2,5-dimercapto-1,3,4-thiadiazole(hereinafter referred to "DMcT") monomer powder were dissolved in 3 g(0.03 mole) of N-methyl-2-pyrrolidone (hereinafter referred to "NMP") toobtain a yellowish transparent viscous DMcT-NMP solution. To thissolution, were added 0.5 g (0.003 mole) of polyaniline powder ("ANILEAD"available from Nitto Denko Corp., Japan) and the combined solution wasthen heated at 80° C. in a sealed container whose inner atmosphere wasreplaced by an inert gas to obtain a black purple non-transparentcomposite product having certain adhesive property. The compositeproduct thus obtained was printed on an electrically-conductive carbonfilm with a thickness of 20 μm, composed of carbon black and afluorocarbon resin, in a manner to give a layer with a thickness of 200μm over the film. The printed film was punched into disks having adiameter of 12.5 mm to give an electrode A.

EXAMPLE 2

By adding 0.5 g (0.003 mole) of polyaniline powder ("ANILEAD" availablefrom Nitto Denko Corp., Japan) to 100 ml of NMP and pouring asupernatant (20 ml) of the mixture over a glass flat dish with adiameter of 90 mm, and then heating the poured supernatant in a vacuumat 60° C. to remove the NMP contained therein, a polyaniline film (0.15g, 0.001 mole) with a thickness of 20 μm was obtained. A DMcT-NMPsolution (DMcT 0.0033 mole and NMP 0.01 mole) obtained by dissolving 1.5g of DMcT monomer powder in 3 g (0.03 mole) of NMP was then spread overthe polyaniline film. The whole was heated at 80° C. in an argonatmosphere to obtain a composite product film with a thickness of 125μm. This composite product film was rolled with the application ofpressure together with a carbon film similar to that of Example 1 toobtain a unitary body and punched into disks having a diameter of 12.5mm to give an electrode B.

COMPARATIVE EXAMPLE 1

Separately, an electrode C having a thickness of 130 μm and a diameterof 12.5 mm was produced by using 0.55 parts by weight of DMcT, 0.35parts by weight of graphite powder and 2.0 parts by weight of a gelelectrolyte, which will be mentioned below.

PRODUCTION OF BATTERY CELLS

First, the above-mentioned gel electrolyte was prepared by gelling 3.0 gof polyacrylonitrile with a mixed solution (1:1 by volume) of propylenecarbonate and ethylene carbonate which dissolved LiBF₄ in 1M. By usingeach of the electrode A and B obtained by Examples 1 and 2, and theelectrode C obtained by Comparative Example 1 as the cathode, a metallithium disk with a thickness of 0.3 mm as the anode, and the abovementioned gel electrolyte as the separator layer which was formed tohave a thickness of 0.6 mm, battery cells A, B or C each having adiameter of 13 mm were produced, respectively.

EVALUATION OF ELECTRODE PERFORMANCE

After charging the battery cells A, B and C at a constant chargingvoltage of 3.5 V at room temperature for 17 hours, each of the batterycells was discharged at a constant discharge current of 1 μA, 10 μA, 100μA, 500 μA or 1 mA for 30 seconds, and the cell voltages at the end ofthe discharging were recorded. The electrode performances were evaluatedwith respect to their current-voltage characteristics of the batterycells. The results of the evaluation were summarized in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        (Voltages of the battery cells, V)                                                   Electrode                                                                                             C                                              Current  A           B         Comparative                                    Value    Example 1   Example 2 Example 1                                      ______________________________________                                         1 μA 3.28        3.20      2.32                                            10 μA                                                                              3.26        3.18      1.85                                           100 μA                                                                              3.22        3.02      1.15                                           500 μA                                                                              3.08        2.88      0.64                                            1 mA    2.82        2.66      0.18                                           ______________________________________                                    

As clearly shown by the above results, it was appreciated that thebattery cells using the electrodes A and B of Examples 1 and 2 producedin accordance with the present invention gave higher cell voltages ascompared with the battery cell using the electrode C of ComparativeExample 1. Taking the cell voltages at the discharging current value of500 μA for comparison, it was found that the electrodes A and B gave ahigher cell voltage by around 2.0 V than that of the electrode C. Thatis, by using an electrode produced in accordance with the presentinvention, it is possible to obtain a battery cell which can beserviceable at a high discharge current.

EXAMPLE 3

One point five (1.5) g (0.01 mole) of DMcT monomer powder were dissolvedin 5 g of NMP to obtain a yellowish transparent viscous DMcT-NMPsolution. To this solution, 2.5 g (0.015 mole) of polyaniline powder ina de-doped and reduced state ("ANILEAD" available from Nitto DenkoCorp., Japan) were added and the combined solution was then heated at80° C. in a sealed container, whose inner atmosphere was replaced by aninert gas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing 2.5 g of V₆ O₁₃ powderhaving a mean particle size of 6 μm in 10 g of NMP, and thoroughlymixing the dispersion. The slurry thus obtained was then added to theabove-mentioned black purple liquid and mixed in a homogenizer at 5000rpm. for about 10 minutes to obtain a dispersion. The dispersion wasthen heated in a rotary evaporator to remove a part of the NMP and tomake it a dispersion having some adhesive property, and printed on anelectrically-conductive carbon film with a thickness of 20 μm, composedof carbon black and a fluorocarbon resin, in a manner to give a layerhaving a thickness of 120 μm over the film. After heating the obtainedfilm for about 30 minutes at 80° C. in a vacuum of 20 cm Hg to removethe NMP contained therein, it was punched into disks having a diameterof 12.5 mm to give a composite electrode D.

EXAMPLE 4

One point five (1.5) g (0.01 mole) of DMcT monomer powder were dissolvedin 5 g of NMP to obtain a yellowish transparent viscous DMcT-NMPsolution. To this solution, 2.5 g (0.015 mole) of polyaniline powder ina de-doped reduced state ("ANILEAD" available from Nitto Denko Corp.,Japan) were added and the combined solution was then heated at 80° C. ina sealed container, whose inner atmosphere was substituted by an inertgas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing and mixing 2.5 g ofLiCoO₂ powder having a mean particle size of 12 μm in 10 g of NMP. Theslurry thus obtained was then added to the above-mentioned black purpleliquid and mixed in a homogenizer at 5000 rpm. for about 10 minutes toobtain a dispersion. The dispersion was then heated in a rotaryevaporator to remove a part of the NMP and to make it a dispersionhaving some adhesive property, and printed on an electrically-conductivecarbon film with a thickness of 20 μm, composed of carbon black and afluorocarbon resin, in a manner to give a layer with a thickness of 120μm over the film. After heating the obtained film for about 30 minutesat 80° C. in a vacuum of 20 cm Hg to remove a part of the NMP containedtherein, it was punched into disks having a diameter of 12.5 mm to givea composite electrode E.

EXAMPLE 5

One point five (1.5) g (0.0087 mole) of DDPy monomer powder weredissolved in 5 g of NMP to obtain a yellowish transparent DDPy-NMPsolution with some viscosity. To this solution, 2.5 g (0.015 mole) ofpolyaniline powder in a de-doped reduced state ("ANILEAD" available fromNitto Denko Corp., Japan) were added and the combined solution was thenheated at 80° C. in a sealed container, whose inner atmosphere: wassubstituted by an inert gas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing and mixing 2.5 g ofLiNiO₂ powder having a mean particle size of 8 μm in 10 g of NMP. Theslurry thus obtained was then added to the above-mentioned black purpleliquid and mixed in a homogenizer at 5000 rpm. for about 10 minutes toobtain a dispersion. The dispersion was then heated in a rotaryevaporator to remove a part of the NMP and to make it a dispersionhaving some adhesivity, and printed on an electrically-conductive carbonfilm with a thickness of 20 μm, composed of carbon black and afluorocarbon resin, in a manner to give a layer with a thickness of 120μm over the film. After heating the obtained film for about 30 minutesat 80° C. in a vacuum of 20 cm Hg to remove the NMP contained therein,it was punched into disks having a diameter of 12.5 mm to give acomposite electrode F.

EXAMPLE 6

One point five (1.5) g (0.0087 mole) of DDPy monomer powder weredissolved in 5 g of NMP to obtain a yellowish transparent viscousDDPy-NMP solution. To this solution, 2.5 g (0.015 mole) of polyanilinepowder in a de-doped reduced state ("ANILEAD" available from Nitto DenkoCorp., Japan) were added and the combined solution was then heated at80° C. in a sealed container, whose inner atmosphere was substituted byan inert gas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing and mixing 2.5 g ofLiMn₂ O₄ powder having a mean particle size of 5 μm in 10 g of NMP. Theslurry thus obtained was then added to the above-mentioned black purpleliquid and mixed in a homogenizer at 5000 rpm. for about 10 minutes toobtain a dispersion. The dispersion was then heated in a rotaryevaporator to remove a part of the NMP and to make it a dispersionhaving some adhesivity, and printed on an electrically-conductive carbonfilm with a thickness of 20 μm, composed of carbon black and afluorocarbon resin, in a manner to give a layer with a thickness of 120μm over the film. After heating the obtained film for about 30 minutesat 80° C. in a vacuum of 20 cm Hg to remove the NMP contained therein,it was punched into disks having a diameter of 12.5 mm to give acomposite electrode G.

EXAMPLE 7

One point five (1.5) g (0.01 mole) of DMcT monomer powder were dissolvedin 5 g of NMP to obtain a yellowish transparent viscous DMcT-NMPsolution. To this solution, 2.5 g (0.015 mole) of polyaniline powder ina de-doped reduced state ("ANILEAD" available from Nitto Denko Corp.,Japan) were added and the combined solution was then heated. at 80° C.in a sealed container, whose inner atmosphere was substituted by aninert gas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing and mixing 2.5 g of V₂O₅ powder having a mean particle size of 15 μm in 10 g of NMP. Theslurry thus obtained was then added to-the above-mentioned black purpleliquid and mixed in a homogenizer at 5000 rpm. for about 10 minutes toobtain a dispersion. The dispersion was then heated in a rotaryevaporator to remove a part of the NMP and to make it a dispersionhaving some adhesivity, and the thus treated dispersion was poured on aglass flat dish to be flown thereover. The poured dispersion was thenheated for about 5 hours at 80° C. under a reduced pressure of a vacuumof 1 cm Hg to obtain a black film. The film thus obtained was pulverizedto give an electrode powder.

Separately, a gel electrolyte was prepared by gelling 3.0 g ofpolyacrylonitrile with a mixed solution (1:1 by volume) of propylenecarbonate and ethylene carbonate which dissolved LiBF₄ in 1M.

A composite electrode H is produced by mixing 5 parts by weight of theabove electrode powder with 4 parts by weight of the gel electrolyte,and then by printing the mixture on the carbon film in a manner to givea layer with a thickness of about 120 μm thereover and by punching theprinted film into disks having a diameter of 12.5 mm.

EXAMPLE 8

One point five (1.5) g (0.0087 mole) of DDPy monomer powder weredissolved in 5 g of NMP to obtain a yellowish transparent viscousDDPy-NMP solution. To this solution, 2.5 g (0.015 mole) of polyanilinepowder in a de-doped reduced state ("ANILEAD" available from Nitto DenkoCorp., Japan) were added and the combined solution was then heated at80° C. in a sealed container, whose inner atmosphere was substituted byan inert gas, to obtain a black purple liquid.

Separately, a slurry was obtained by dispersing and mixing 2.5 g of V₆O₁₃ powder having a mean particle size of 6 μm in 10 g of NMP. Theslurry thus obtained was then added to the above-mentioned black purpleliquid and mixed in a homogenizer at 5000 rpm. for about 10 minutes toobtain a dispersion. The dispersion was then heated in a rotaryevaporator to remove a part of the NMP and to make it a dispersionhaving some adhesivity, and the thus treated dispersion was poured on aglass flat dish to be flown thereover. The poured dispersion was thenheated for about 5 hours at 80° C. under a reduced pressure of a vacuumof 1 cm Hg to obtain a black film. The film thus obtained was pulverizedto give an electrode powder which was then finished to a compositeelectrode I in a manner similar to that in Example 7.

PRODUCTION OF BATTERY CELLS

By using the composite electrodes D, E, F, G, H and I obtained byExamples 3, 4, 5, 6, 7 and 8, as the cathodes, metal lithium disks witha thickness of 0.3 mm as the anodes, and the above mentioned gelelectrolyte as the separator layer with a thickness of 0.6 mm, batterycells D, E, F, G, H and I each having a diameter of 13 mm were produced,respectively.

COMPARATIVE EXAMPLE 2

Separately, an electrode J having a thickness of 120 μm and a diameterof 12.5 mm was produced by mixing 5 parts by weight of mixed powdercomposed of 2.5 g of polyaniline powder which was the same as that usedin Examples 3-8, and 2.5 g of V₂ O₅ powder having a mean particle sizeof 15 μm, with 4 parts by weight of the above-mentioned gel electrolyte.The mixture was printed on a carbon film in a manner to give a layer ofabout 120 μm thickness and then punched into disks. Using the electrodeJ, a battery cell J was produced in a similar manner to those of thebattery cells D-I above.

COMPARATIVE EXAMPLE 3

An electrode K having a thickness of 120 μm and a diameter of 12.5 mmwas produced by mixing 5 parts by weight of mixed powder composed of 2.5g of the same polyaniline powder and 2.5 g of V₆ O₁₃ powder having amean particle size of 6 μm, with 4 parts by weight of theabove-mentioned gel electrolyte. The mixture was printed on a carbonfilm in a manner to give a layer of about 120 μm thickness and thenpunched into disks. Using the electrode K, a battery cell K was producedin a similar manner to those of the battery cells. D-I above.

COMPARATIVE EXAMPLE 4

Separately, an electrode L having a thickness of 120 μm and a diameterof 12.5 mm was produced by pouring a solution composed of 5 g of NMP and0.2 g of polyaniline dissolved in the former over a glass flat dish andrepeating an operation of removing the NMP by drying the solution for 8times at 80° C. under a reduced pressure of a vacuum of 1 cm Hg toobtain an electrode comprising polyaniline film having a thickness of120 μm. Using the electrode L, a battery cell L was produced in asimilar manner to those of the battery cells D-I above.

EVALUATION OF ELECTRODE PERFORMANCE

After charging the battery cells D, E, F, G, H, I, J, K and L at aconstant charging voltage of 4.3 V at room temperature for 17 hours,each of the battery cells was discharged at a constant discharge currentof 1 μA, 10 μA, 100 μA, 500 μA or 1 mA for 30 seconds, and the cellvoltages at the end of the discharging were recorded. The electrodeperformances were evaluated with respect to the current-voltagecharacteristics of the battery cells. The results of the evaluation weresummarized in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        (Voltages of the battery cells, V)                                            Cell Current                                                                  value     1 μA                                                                              10 μA 100 μA                                                                            500 μA                                                                           1 mA                                   ______________________________________                                        Examples                                                                      D         3.95   3.55     3.22   3.05  2.85                                   E         4.10   3.96     3.72   3.58  3.41                                   F         4.15   4.02     3.86   3.65  3.43                                   G         4.25   4.12     3.96   3.82  3.58                                   H         3.85   3.45     3.02   2.80  2.61                                   I         4.00   3.65     3.32   3.02  2.82                                   Comparative                                                                   examples                                                                      J         3.52   3.25     2.85   2.25  1.98                                   K         3.61   3.26     3.05   2.47  2.12                                   L         3.55   3.46     3.15   3.05  2.55                                   ______________________________________                                    

Further, after charging the battery cells D, E, F, G, H, I, J, K and Lat a constant charging voltage of 4.3 V at room temperature for 17hours, each of the battery cells was continuously discharged at aconstant discharge current of 270 μA, and each of the service capacitiesof the battery cells, after each of the cell voltages became down to 1.5V, was measured. The results of the service capacity measurements weresummarized in Table 3 below.

                  TABLE 3                                                         ______________________________________                                        Service Capacity (mAh)                                                               Battery Cell                                                                  D    E     F      G   H   I    J   K    L                              ______________________________________                                        Service  3.5    2.5   2.8  2.1 3.0 3.6  2.5 2.8  1.7                          capacity                                                                      ______________________________________                                    

As clearly shown by the above mentioned results, it was appreciated thatthe battery cells D, E, F, G, H and I produced in accordance with thepresent invention each gave a higher cell voltage as compared with thebattery cells J, K and L of the comparative examples. It was furtherappreciated that the battery cells of the present invention were able tooperate at a larger discharge current and had larger capacities thanthose of the comparative examples.

As has been discussed in the foregoing, the composite electrode, whichcombines the organic disulfide compound, polyaniline andN-methyl-2-pyrrolidone, and is produced in accordance with the presentinvention, makes the battery operation at a larger current possible,which has been difficult for the conventional electrode comprising theorganic disulfide compound solely.

The other composite electrode, which combines the organic disulfidecompound, polyaniline and metal oxide, and is produced in accordancewith the present invention, gives a battery capacity 1.5-2 times aslarge as compared with the electrode composed solely of polyaniline. Italso makes the operation with a small voltage drop, i.e., in a lowpolarization, possible. The operation with a small voltage drop has beendifficult for the conventional electrode comprising polyaniline andmetal oxide.

In the foregoing embodiments, although the results of the evaluations ofthe electrode performances by using the battery cells which uses metallithium as the anode, it is also possible to configure an electrochromicdisplay device having a rapid coloring and decoloring speeds, and abiochemical sensor such as a glucose sensor having a rapid response aswell as an electrochemical analog memory which has rapid write-in andread-out speeds by employing the electrode in accordance with thepresent invention as the counter electrode.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosures are not to be interpreted as limiting. Various alterationsand modifications will no doubt become apparent to those skilled in theart to which the present invention pertains, after having read the abovedisclosure.

What is claimed is:
 1. A composite electrode comprising:an organicdisulfide compound which contains at least one sulfur-sulfur bond or atleast two thiolate or thiol groups, wherein said sulfur-sulfur bond iscleaved when electrolytically reduced to form thiolate groups or thiolgroups and said sulfur-sulfur bond is regenerated when said thiolate orthiol groups are electrolytically oxidized, N-alkyl-2-pyrrolidone or2-pyrrolidone represented by the formula: ##STR2## where R represents ahydrogen atom or an alkyl group, and polyaniline.
 2. The compositeelectrode according to claim 1, wherein said organic disulfide compoundis at least one selected from the group consisting of dithioglycol(HSCH₂ CH₂ SH), 2,5-dimercapto-1,3,4-thiadiazole (C₂ N₂ S(SH)₂),s-triazine-2,4,6-trithiol (C₃ H₃ N₃ S₃),7-methyl-2,6,8-trimercaptopurine (C₆ H₆ N₄ S₃), and4,5-diamino-2,6-dimercaptopyrimidine (C₄ H₆ N₄ S₂).
 3. A compositeelectrode comprising:an organic disulfide compound which contains atleast one sulfur-sulfur bond or at least two thiolate or thiol groups,wherein said sulfur-sulfur bond is cleaved when electrolytically reducedto form thiolate groups or thiol groups and said sulfur-sulfur bond isregenerated when said thiolate or thiol groups are electrolyticallyoxidized, N-alkyl-2-pyrrolidone or 2-pyrrolidone represented by theformula: ##STR3## where R represents a hydrogen atom or an alkyl group,polyaniline, and a metal oxide powder.
 4. The composite electrodeaccording to claim 1 or 3, wherein said polyaniline is a polyaniline ofde-doped and reduced state.
 5. The composite electrode according toclaim 4, wherein said metal oxide is at least one of transition metaloxide selected from the group consisting of LiCoO₂, V₆ O¹³, LiMn₂ O₄, V₂O₅ and LiNiO₂.
 6. A composite electrode comprising:an organic disulfidecompound which contains at least one sulfur-sulfur bond or at least twothiolate or thiol groups, wherein said sulfur-sulfur bond is cleavedwhen electrolytically reduced to form thiolate groups or thiol groupsand said sulfur-sulfur bond is regenerated when said thiolate or thiolgroups are electrolytically oxidized, polyaniline, and a metal oxidepowder which is homogeneously mixed with said organic disulfide compoundand said polyaniline.
 7. The composite electrode according to claim 6,wherein said polyaniline is a polyaniline of de-doped and reduced state.8. The composite electrode according to claim 6, wherein said metaloxide is at least one of transition metal oxide selected from the groupconsisting of LiCoO₂, V₆ O₁₃, LiMn₂ O₄, V₂ O₅ and LiNiO₂.
 9. A lithiumsecondary battery comprising:an electrolyte; an anode; and a compositecathode comprising:an organic disulfide compound which contains at leastone sulfur-sulfur bond or at least two thiolate or thiol groups, whereinsaid sulfur-sulfur bond is cleaved when electrolytically reduced to formthiolate groups or thiol groups and said sulfur-sulfur bond isregenerated when said thiolate or thiol groups are electrolyticallyoxidized; polyaniline, wherein said organic disulfide compound and saidpolyaniline are homogeneously mixed; and N-alkyl-2-pyrrolidone or2-polyrrolidone represented by the formula: ##STR4## where R representsa hydrogen atom or an alkyl atom.
 10. The lithium secondary batterycomprising a cathode, an electrolyte and an anode, wherein the cathodeis a composite electrode, which comprises:an organic disulfide compoundwhich contains at least one sulfur-sulfur bond or at least two thiolateor thiol groups wherein said sulfur-sulfur bond is cleaved whenelectrolytically reduced to form thiolate groups or thiol groups andsaid sulfur-sulfur bond is regenerated when said thiolate or thiolgroups are electrolytically oxidized; polyaniline; and a metal oxidepowder which is homogeneously mixed with said organic disulfide compoundand said polyaniline.
 11. The composite electrode according to claim 10,wherein said metal oxide is at least one of transition metal oxideselected from the group consisting of LiCoO₂, V₆ O₁₃, LiMn₂ O₄, V₂ O₅and LiNiO₂.
 12. A composite electrode comprising:an organic disulfidecompound which contains at least one sulfur-sulfur bond or at least twothiolate or thiol groups, wherein said sulfur-sulfur bond is cleavedwhen electrolytically reduced to form thiolate groups or thiol groupsand said sulfur-sulfur bond is regenerated when said thiolate or thiolgroups are electrolytically oxidized, and polyaniline which ishomogeneously mixed with said organic disulfide compound, saidpolyaniline and said organic disulfide being homogeneously mixed by theprocess comprising the steps of: dissolving said organic disulfidecompound and said polyaniline in a solvent; and removing said solvent toobtain said homogenous mixture.
 13. A lithium secondary batterycomprising a cathode, an electrolyte, and an anode, wherein said cathodeis a composite electrode comprising:an organic disulfide compound whichcontains at least one sulfur-sulfur bond or at least two thiolate orthiol groups, wherein said sulfur-sulfur bond is cleaved whenelectrolytically reduced to form thiolate groups or thiol groups andsaid sulfur-sulfur bond is regenerated when said thiolate or thiolgroups are electrolytically oxidized, and polyaniline which ishomogeneously mixed with said organic disulfide compound, saidpolyaniline and said organic disulfide being homogeneously mixed by theprocess comprising the steps of: dissolving said organic disulfidecompound and said polyaniline in a solvent; and removing said solvent toobtain said homogenous mixture.